1. Field of the Invention
The present invention relates to air moving devices, and in particular, to blowers of the type which are used with high efficiency (e.g., 90% or higher efficiency) furnaces for drawing air from outside of a building into the furnace to support combustion and to expel combustion exhaust products outside of the building. More particularly, the present invention relates to a blower which provides increased air flow through the blower and decreased blower noise while maintaining an overall diameter for the housing of the blower which conforms to, and is compatible with, industry standard mounting bolt patterns on furnaces for attachment of blower housings.
2. Description of the Related Art
In high efficiency furnaces, standard chimney air-draw effects are not sufficient to assure the required air flow through the furnace heat exchangers, and therefore, high efficiency furnaces utilize draft inducer blowers to provide sufficient air flow through the furnace. In particular, the blowers of high efficiency furnaces pull flue gases through the furnace heat exchangers and then push the flue gases out through exhaust piping to the exterior of the building. The length of the flue piping is limited by the static pressure induced on the flue gases by the draft inducer blower, and higher static pressures typically allow longer runs of flue piping. One measure of the efficiency of the draft inducer blower is the static pressure generated by the blower on the flue gases at a given air flow rate, wherein a blower is more efficient if it can generate higher pressures and air flows for a given power input to the electric motor which drives the blower impeller.
Centrifugal blowers generate pressure by doing work on the air flow through the blower housing by rotating the impeller of the blower. The angular momentum of the impeller produces a velocity pressure within the blower housing that must be converted to a static pressure by diffusion. In blowers where the diffuser section is wrapped around the periphery of the impeller, the diffuser may take the form of a scroll or volute which increases in the radial direction with respect to the rotational axis of the impeller. Forward-bladed impellers common in known furnace blowers require a volute diffuser section to convert velocity pressure into static pressure. Ideally, if the diffuser section grows at the same rate as the airflow being radially pumped into the diffuser section from the impeller, the airflow through all of the impeller blade passages will be uniform, and the airflow around the volute diffuser section will have uniform average velocity.
For example, in one known expanding scroll-type diffuser blower disclosed in U.S. Pat. No. 4,599,042 to Colliver, the axial end walls of the blower housing are parallel to one another, and the outer or side wall of the blower housing is scrolled radially outwardly such that the radial distance between the axis of the impeller and the side wall progressively increases at a constant rate around the blower circumference from the cutoff region of the blower housing toward the outlet of the blower housing.
However, in draft inducer blowers for high efficiency furnaces, the standard bolt pattern in the wall of the furnace to which the blower housing is attached, imposes a limitation to the diameter and overall size of the blower housing in the radial dimension. Also, due to the potential for corrosion of the attachment bolts by the exhaust flue gases, the side wall of the blower housing is usually positioned between the attachment bolts and the interior of the blower housing. For these reasons, the effective air volume of the blower is generally restrained in the radial dimension by the standard bolt pattern of existing furnaces.
One known blower for a high efficiency furnace is shown in FIGS. 1-4, and generally includes a blower housing 20 having a housing body 22 and a housing cover 24. Housing body 22 is formed as a molded plastic component, having a cylindrical outer wall 26, a planar top wall 28, and an axially recessed, planar wall 30 to which electric motor 32 is mounted. Housing body 22 further includes an integral, tubular exhaust transition 34 and outlet projecting tangentially therefrom, to which an exhaust pipe (not shown) is connected. Housing cover 24 is a substantially flat, molded plastic circular plate which is attached to housing body 22 by being captured between housing body 22 and wall 36 of a furnace, as shown in FIG. 4. Specifically, after blower housing 20 is positioned near the furnace wall 36 as shown in the left side of FIG. 4, a plurality of bolts 38 are inserted through respective mounting lugs 40 in housing body 22 and into a set of corresponding holes 42 in furnace wall 36 to thereby attach the blower housing 20 to the furnace, as shown in the right side of FIG. 4. Holes 42 in furnace wall 36 are disposed in a standard pattern with a predetermined, fixed diameter.
An impeller 44, shown in FIGS. 2-4, is disposed within the interior of blower housing 20 between housing body 22 and housing cover 24, and is mounted for rotation upon drive shaft 46 of motor 32. In operation, rotation of impeller 44 by motor 32 draws exhaust gases through a centrally disposed circular inlet 48 in housing cover 24 from the furnace into the blower housing 20, and the exhaust gases are discharged through the outlet of exhaust transition 34. Although the foregoing blower housing has proven to be effective for use with high efficiency furnaces, improvements to same are desired.
In blower housing 20, the diameter of outer wall 26 and the corresponding radial dimension of blower housing 20 is limited by the standard bolt pattern of the furnace. Therefore, forming outer wall 26 to create a radially-expanding diffuser section, in which the distance between the axis of impeller 44 and outer wall 26 constantly increases in the radial direction around the circumference of blower housing from cutoff 50 (FIG. 2) toward exhaust transition 34, is not practicable for converting the velocity pressure of the air flow into static pressure. Thus, because the size of blower housing 20 is effectively fixed in the radial direction, other means of diffusing the velocity pressure of the air flow must be utilized. A further, related design consideration is the desirability to maximize the diameter and overall size of the impeller used with the blower housing.
As described below, the cylindrical outer wall 26 of blower housing 20 effectively sets up a diffusion section within the air flow so that blower housing 20 can accommodate the radial air flow from impeller 44. Referring to FIG. 2, air flow within blower housing is shown by the several arrows. The cylindrical outer wall 26 of blower housing 20 causes the air flow to have a high pressure and very low or no velocity flow from cutoff 50 of blower housing 20 to about a third of the way around the circumference of outer wall 26. As shown by the air flow arrows in FIG. 2, some air flow may actually go backwards from this high pressure region toward cutoff 50 and exhaust transition 34. Air flow tends to stagnate in the blade passages of impeller 44 as the impeller blades pass this high pressure area. About a third of the way around the circumference of outer wall 26 from cutoff 50, the air starts to move and then accelerate around the remainder of the circumference of blower housing 20 towards the outlet of exhaust transition 34. Thus, as shown by the air flow arrows in FIG. 2, the air flow through the blade passages of impeller 44 only exits the blade passages in about two-thirds of the blade passages of impeller 44 at any given time and, as impeller 44 rotates, the air flow in the blade passages thereof is inefficiently surging to full flow and then back to a stop during every revolution of impeller 44. In this manner, blower housing 20 sets up its own “diffuser” section by creating asymmetric, cyclic flow through the blade passages of impeller 44, which is not optimally efficient.
A further disadvantage with known blower housings for high efficiency furnaces is the presence of a rather large gap between housing cover 24 and the bottom of impeller 44, shown as distance D1 in FIGS. 3 and 4, which is typically approximately 0.257 inches. This gap is necessary to allow for some inward deflection of housing cover 24, as shown in the right side of FIG. 4, when blower housing 20 is attached to wall 36 of a furnace, in which the inlet 48 of housing cover 24 may be deflected upwardly toward impeller 44 by contact with gasket 52 between housing cover 24 and furnace wall 36. Typically, distance D1 is reduced to 0.247 inches or less after such deflection. The relatively large gap between housing cover 24 and impeller 44 could potentially allow some recirculation of the air flow within blower housing 20, in which air leaks back between impeller 44 and housing cover 24 toward inlet 48 of blower housing 20 instead of exiting through the outlet of exhaust transition 34, which could potentially lessen the performance and efficiency of the blower. Additionally, a large degree of deflection of the inlet 48 of housing cover 24 toward impeller 44 could potentially inhibit airflow through inlet 48 to the central inlet portion of impeller 44. Specifically, as shown in FIG. 4, distance D2 between housing cover 24 and impeller 44 near inlet 48 reduces from approximately 0.297 inches, as shown in the left side of FIG. 4, to approximately 0.120 inches, as shown in the right side of FIG. 4, by inward deflection of housing cover 24.
What is needed is a draft inducer blower housing for high efficiency furnaces which is an improvement on the foregoing.